1. Field of the Invention
The invention relates to method for numerically analyzing a flow field and a temperature field of fluid in a vehicle drive unit. More particularly, the invention relates to technology for quickly performing that numerical analysis.
2. Description of the Related Art
Japanese Patent Application Publication No. 4-15761 (JP-A-4-15761), International Publication No. WO 2004/061723A1, and Japanese Patent Application Publication No. 2002-117018 (JP-A-2002-117018) and the like describe art related to the invention. Of these, JP-A-4-15761 describes an analysis method that uses a CAE/CAD/CAM system that integrates the heat flux and obtains the temperature change in a cold cycle structure in which the heat flux and the mold temperature changes from moment to moment, based on a three-dimensional analytical model of that cold cycle structure. For example, the analysis method described above may be applied to CAE analysis of a cold cycle of an injection mold, or the like.
The analysis method described in JP-A-4-15761 takes into account the two elements of heat transfer and free surface that is the surface boundary of molten resin or the like with respect to the air. Here, CAE analysis (numerical analysis) may be performed on a vehicle drive unit that includes a moving body that moves inside of a case, and a cooling medium that is a fluid that fills a portion of the inside of the case and contacts at least a portion of the moving body. With this vehicle drive unit, because the cooling medium does not fill up the entire whole of the inside of the case, it has a free surface that is not constrained by the inside wall of the case. The cooling medium is agitated or the like by the movement of the moving body. Therefore, when numerically analyzing the flow of the cooling medium and heat transfer in the vehicle drive unit in which heat from the moving body is dissipated outside the case, it is necessary to take the three elements of free surface, heat transfer, and the movement of the moving body, e.g., the rotational transfer of the gears, into account. Therefore, just like the analysis method described in JP-A-4-15761 that only takes the two elements of free surface and heat transfer into account, numerical analysis is unable to be performed. As a result, in the vehicle drive unit, the calculation load of the numerical analysis on the electronic calculator becomes enormous, so the time required to perform the numerical analysis becomes extremely long. This is thought to be because the flow of the cooling medium that is agitated or the like by the movement of the moving body reaches a steady state in a short elapsed time of that system, but it takes a long time for the temperature of the cooling medium to reach a steady state, i.e., the time scales of these are very different from each other.
Also, as shown in the last row in FIG. 8, conventionally CAE analysis (numerical analysis) has been performed on the oil flow in a drive system. However, while that CAE analysis does take the free surface and rotational transfer into account, it does not take heat transfer into account. Also, as CAE analysis performed conventionally in another technical field, there is CAE analysis of vehicle wind flow for evaluating the cooling performance of the brakes that brake the wheels of the vehicle, for example. However, as shown in the first row in FIG. 8, with CAE analysis for vehicle wind flow, while the heat transfer and the rotational transfer of the wheels and the brake discs and the like are taken into account, it is not necessary to take the free surface of the fluid into account. That is, as shown in FIG. 8, the related CAE analysis does not take into account all three of the elements of the free surface of the fluid, heat transfer, and rotational transfer of the moving body as the phenomena to be analyzed. Also, supposing that CAE analysis is performed on the temperature change of a liquid that fills a portion of the inside of a hollow pipe when that liquid flows from one end to the other end of that hollow pipe, the three elements of free surface, heat transfer, and flowrate are taken into account. However, although the flowrate of the liquid inside the hollow pipe at that time does not change over time, even if the free surface of the cooling medium is stable in the vehicle drive unit, the flowrate within the cooling medium changes over time due to the movement of the moving body, so CAE analysis for the hollow pipe could not be applied to the vehicle drive unit.
In view of this, when performing CAE analysis on cooling medium flow or heat transfer in a vehicle drive unit, the inventor came up with an undisclosed CAE analysis method (i.e., numerical analysis method) by which numerical analysis of the flow field of the cooling medium is performed separately from numerical analysis of the temperature field of the cooling medium, in order to shorten the time required to perform the CAE analysis. Using a rotor of an electric motor that generates heat by conducting electricity and rotates about an axis as the moving object, and cooling oil that contacts a portion of the rotor as the cooling medium, for example, this numerical analysis method numerically analyzes the flow field that takes into account the free surface of the cooling oil and the rotational transfer of the rotor, separately from the temperature field that takes into account the heat transfer, as shown in FIG. 9. In FIG. 9, in STEP [1], i.e., the first step, the numerical analysis for only the flow field is conducted until that flow field becomes quasisteady. Then in STEP [2], i.e., the second step, the flow field is maintained as it is after STEP [1] ends and numerical analysis for only the temperature field is conducted until the temperature field reaches thermal equilibrium. Then finally in STEP [3], i.e., the third step, numerical analysis of the flow field and numerical analysis of the temperature field are conducted together. Here, the term quasisteady above refers to the fact that on a microscopic level, the flow of the cooling medium while it is being agitated is not steady but rather is changing over time, while on a macroscopic level, the liquid surface is stable and does not change substantially over time.
Upon performing numerical analysis according to the steps in FIG. 9, in STEP [2], the total amount of heat dissipated in the temperature field becomes larger with respect to the amount of heat generated as the number of calculation cycles of the numerical analysis increases, i.e., as the set time passes in the numerical analysis, as shown in FIG. 10, so the temperature field is unable to reach thermal equilibrium in STEP [2]. This is thought to be due to the fact that the flow field and the temperature field are analyzed separately in this numerical analysis even though there is a correlation between the flow field and the temperature field. That is, a change in the temperature of the cooling oil will cause a change in the viscosity of the cooling oil, which in turn will cause a change in the flow of the cooling oil, and a change in the flow of the cooling oil will change the ease with which heat is transferred between the cooling oil and the case or the rotor. As a result, adequate results may not be able to be obtained when numerical analysis of the flow field and numerical analysis of the temperature field are simply performed separately.